Table Of Contents

Configuring Spanning-Tree Bridging for the CSS

CSS Spanning-Tree Bridging Quick Start

Configuring Spanning-Tree Bridge Aging-Time

Configuring Spanning-Tree Bridge Forward-Time

Configuring Spanning-Tree Bridge Hello-Time

Configuring Spanning-Tree Bridge Max-Age

Configuring Spanning-Tree Bridge Priority

Disabling Bridge Spanning-Tree

Showing Bridge Configurations

Configuring Spanning-Tree Bridging for the CSS

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The CSS supports configuration of Spanning-Tree Protocol (STP) bridging. Spanning-tree bridging detects, and then prevents, loops in the network. Use the bridge command to configure global spanning-tree bridging options for the CSS, such as bridge aging time, forward delay time, hello time interval, and maximum age. Make sure you configure the spanning-tree bridging parameters the same on all switches running STP in the network.

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Note When connecting a Cisco Catalyst switch to a CSS using an 802.1Q trunk and the STP, the Catalyst runs a spanning-tree instance for each VLAN. When you configure an 802.1Q trunk on an Ethernet interface for the Catalyst switch, the bridge protocol data units (BPDUs) are tagged with the corresponding VLAN ID and the destination MAC address changes from the standard 01-80-C2-00-00-00 to the proprietary 01-00-0c-cc-cc-cd. This modification allows Cisco switches operating in a non-Cisco (a mix of other vendors) 802.1Q trunk environment to maintain spanning-tree states for all VLANs. Although the CSS maintains a spanning-tree instance for each VLAN as well, the CSS uses the standard 01-80-C2-00-00-00 destination MAC address for all BPDUs (tagged or untagged). When you connect a Cisco Catalyst switch to a CSS over an 802.1Q trunk, the result is that neither switch recognizes the other's BPDUs, and both assume root status. If a spanning-tree loop is detected, the Catalyst switch goes into blocking mode on one of its looped ports.

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This chapter contains the following major sections:

•CSS Spanning-Tree Bridging Quick Start

•Configuring Spanning-Tree Bridge Aging-Time

•Configuring Spanning-Tree Bridge Forward-Time

•Configuring Spanning-Tree Bridge Hello-Time

•Configuring Spanning-Tree Bridge Max-Age

•Configuring Spanning-Tree Bridge Priority

•Disabling Bridge Spanning-Tree

•Showing Bridge Configurations

For details about configuring spanning-tree bridging parameter for an Ethernet interface or for a trunked Ethernet interface and VLAN pair, refer to Chapter 1, Configuring Interfaces and Circuits.

CSS Spanning-Tree Bridging Quick Start

Table 2-1 provides a quick overview of the steps required to globally configure spanning-tree bridging for the CSS. Each step includes the CLI command required to complete the task. For a complete description of each feature and all the options associated with the CLI command, see the sections following Table 2-1.

Table 2-1 Spanning-Tree Bridging Configuration Quick Start 

Task and Command Example
1. Set the bridge filtering database aging time, in seconds, for the CSS. (config)# bridge aging-time 600
2. Set the bridge forward delay time, in seconds, that the bridge uses when acting as the root. (config)# bridge forward-time 9
3. Set the bridge hello time interval, in seconds, that the bridge waits before sending a hello packet. (config)# bridge hello-time 9
4. Set the bridge spanning-tree maximum age, in seconds. (config)# bridge max-age 21
5. Set the priority that the bridge spanning tree uses to choose the root bridge in the network. (config)# bridge priority 1700
6. (Recommended) Display bridge forwarding information. (config)# show bridge status

The following running-configuration example shows the results of entering the commands in Table 2-1.

!*************************** GLOBAL ***************************

  bridge aging-time 600 

  bridge forward-time 9 

  bridge hello-time 9 

  bridge max-age 21 

  bridge priority 1700 

Configuring Spanning-Tree Bridge Aging-Time

The aging time is the timeout period, in seconds, for aging out dynamically learned forwarding information. By default, the bridge filtering database aging time for the CSS is 300 seconds. To set the bridge filtering database aging time for the CSS., use the bridge aging-time command. Enter an integer from 10 to 1000000.

To set the bridge aging time to 600, enter:

(config)# bridge aging-time 600

To restore the default aging time of 300, enter:

(config)# no bridge aging-time

Configuring Spanning-Tree Bridge Forward-Time

The forward time is the delay time, in seconds, that all bridges use for forward delay when this bridge is acting as the root. By default, the bridge forward delay time is 4 seconds. Use the bridge forward-time command to set the bridge forward delay time. Enter an integer from 4 to 30.

To set the bridge forward time to 9, enter:

(config)# bridge forward-time 9

To restore the default delay time of 4, enter:

(config)# no bridge forward-time

Configuring Spanning-Tree Bridge Hello-Time

The hello time is the time, in seconds, that all bridges wait before sending a hello packet (when the bridge acts as the root). By default, the bridge hello time interval is 1 second. Use the bridge hello-time command to set the bridge hello time interval. Enter an integer from 1 to 10.

To set the bridge hello time to 9, enter:

(config)# bridge hello-time 9

To restore the default hello time interval of 1, enter:

(config)# no bridge hello-time

Configuring Spanning-Tree Bridge Max-Age

The maximum age is the time, in seconds, that protocol information received on a port is stored by the CSS (when a bridge acts as the root). By default, the bridge spanning-tree maximum age is 6 seconds. Use the bridge max-age command to set the bridge spanning-tree maximum age. Enter an integer from 6 to 40.

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Note Ensure the bridge maximum age is greater than or equal to 2 times (bridge hello-time + 1 second) and less than or equal to 2 times (bridge forward-time - 1 second).

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To set the bridge maximum age to 21, enter:

(config)# bridge max-age 21

To restore the default maximum age of 6, enter:

(config)# no bridge max-age

Configuring Spanning-Tree Bridge Priority

In spanning tree, the 2-octet field is prepended to the 6-octet MAC address to form an 8-octet bridge identifier. The device with the lowest bridge identifier is considered the highest priority bridge and becomes the root bridge. By default, the bridge priority is set to 32768. Use the bridge priority command to set the priority that the bridge spanning tree uses to choose the root bridge in the network. The range for bridge priority is 0 to 65535.

For example:

(config)# bridge priority 1700

To restore the bridge priority to the default of 32768, enter:

(config)# no bridge priority

Disabling Bridge Spanning-Tree

Spanning-tree bridging is enabled by default. When you disable spanning-tree bridging, the CSS drops those bridge protocol data units (BPDUs) that it recognizes as BPDUs, but forwards the Cisco Systems 802.1Q BPDUs (tagged with the proprietary 01-00-0c-cc-cc-cc-cd destination MAC address) on an 802.1Q VLAN trunk. The CSS can still operate in an 802.1Q spanning-tree environment as long as you do not require that the CSS put any of its ports into a blocking state.

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Caution Disabling spanning-tree bridging may make your network susceptible to packet storms.

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To disable spanning-tree bridging, enter:

(config)# bridge spanning-tree disable

To reenable spanning-tree bridging, enter:

(config)# bridge spanning-tree enable

Showing Bridge Configurations

Use the show bridge forwarding command to display bridge forwarding information. Table 2-2 describes the fields in the show bridge forwarding command output.

Table 2-2 Field Descriptions for the show bridge forwarding Command

Field	Description
VLAN	The bridge interface virtual LAN number
MAC Address	The MAC address for the entries
Port Number	The port number used for bridge forwarding

Use the show bridge status command to display bridge status information. Table 2-3 describes the fields in the show bridge status output.

Table 2-3 Field Descriptions for the show bridge status Command 

Field	Description
STP State	The state of the Spanning-Tree Protocol: Enabled or Disabled.
Root Max Age	The timeout period, in seconds, during which the host times out root information.
Root Hello Time	The interval, in seconds, during which the root bridge broadcasts its hello message to other devices.
Root Fwd Delay	The delay time, in seconds, that the root bridge uses for forward delay.
Designated Root	The bridge ID for the designated root.
Bridge ID	The bridge ID of the bridge.
Port	The port ID.
State	The state of the port. The possible states are as follows: •Block - The blocking state. A port enters the blocking state after CSS initialization. The port does not participate in frame forwarding. •Listen - The listening state. This state is the first transitional state a port enters after the blocking state. The port enters this state when STP determines that the port should participate in frame forwarding. •Learn - The learning state. The port enters the learning state from the listening state. The port in the learning state prepares to participate in frame forwarding. •Forward - The forwarding state. The port enters the forwarding state from the learning state. A port in the forwarding state forwards frames. •Disabled - The disabled state. A port in the disabled state does not participate in frame forwarding or the Spanning-Tree Protocol. A port in the disabled state is non operational.
Designated Bridge	The bridge ID for the designated bridge.
Designated Root	The bridge ID for the designated root.
Root Cost	The cost of the root.
Port Cost	The cost of the port.
Desg Port	Designated port.